Aldol reactions in polypropionate synthesis: High π-face selectivity of enol borinates from α-chiral methyl and ethyl ketones under substrate control

Synthesis of two diastereomeric C1-C22 fragments of spirastrellolide A

The optimisation of a synthetic strategy towards the ABC segment of the cytotoxic macrolide spirastrellolide A is reported, together with its application to the synthesis of two diastereomeric C(1)-C(22) fragments for stereochemical correlation purposes with a putative spirastrellolide degradation product.

Progress toward a total synthesis of spirastrellolide A

Progress toward a total synthesis of spirastrellolide A, a 38-membered marine macrolide, is reported. Syntheses of two diastereomers of the C1-C25 region, and an evolving Sharpless dihydroxylation strategy toward a C26-C40 fragment, are described. The syntheses exploit boron-mediated aldol chemistry to install key stereocenters, and feature late-stage thermodynamically controlled spiroacetalizations.

Toward the synthesis of spirastrellolide A: construction of two C1-C25 diastereomers containing the BC-spiroacetal

[reaction: see text] Stereo-controlled syntheses of two possible C1-C25 diastereomers of spirastrellolide A containing the cis-disubstituted tetrahydropyran and [6,6]-spiroacetal are reported, exploiting boron-mediated asymmetric aldol and allylation methodology.

Synthesis of the C11-C29 fragment of amphidinolide F

[reaction: see text] An efficient synthesis of the C(11)-C(29) fragment 31 of amphidinolide F has been accomplished via a diastereoselective [3 + 2]-annulation reaction of allylsilane 5 and ethyl glyoxylate to prepare the key tetrahydrofuran 15 and a highly stereoselective methyl ketone aldol reaction to generate the C(11)-C(16) segment.

Synthesis of a versatile multifunctional building block for the construction of polyketide natural products containing ethyl side-chains

The synthesis of a multifunctional building block for polyketide construction and several subsequent reactions are presented.

Development of a Third-Generation Total Synthesis of (+)-Discodermolide: An Expedient Still−Gennari-Type Fragment Coupling Utilizing an Advanced β-Ketophosphonate

[structure: see text] A novel total synthesis of the complex polyketide discodermolide, a promising anticancer agent of marine sponge origin, has been completed in 11.1% overall yield over 21 linear steps. This third-generation approach features an unprecedented Still-Gennari-type HWE olefination reaction between advanced C1-C8 beta-ketophosphonate 61 and C9-C24 aldehyde 7, introducing the (8Z)-alkene with 10:1 selectivity. The stereotetrad found in the C1-C8 subunit 61 was established via a highly diastereoselective boron-mediated aldol reaction/in situ reduction between ketone (S)-8 and 3-benzyloxypropanal. The (7S)-configuration was installed by the reduction of enone 73 with K-Selectride.

A second-generation total synthesis of (+)-discodermolide: the development of a practical route using solely substrate-based stereocontrol

A novel total synthesis of the complex polyketide (+)-discodermolide, a promising anticancer agent of sponge origin, has been completed in 7.8% overall yield over 24 linear steps, with 35 steps altogether. This second-generation approach was designed to rely solely on substrate control for introduction of the required stereochemistry, eliminating the use of all chiral reagents or auxiliaries. The common 1,2-anti-2,3-syn stereotriad found in each of three subunits, aldehyde 9 (C(1)-C(5)), ester 40 (C(9)-C(16)), and aldehyde 13 (C(17)-C(24)), was established via a boron-mediated aldol reaction of ethyl ketone 15 and formaldehyde, followed by hydroxyl-directed reduction to give 1,3-diol 14. Alternatively, a surrogate aldehyde 22 was employed for formaldehyde in this aldol reaction, leading to the beta-hydroxy aldehyde 20 as a common building block, corresponding to the discodermolide stereotriad. Key fragment unions were achieved by a lithium-mediated anti aldol reaction of ester 40 and aldehyde 13 under Felkin-Anh control to provide (16S,17S)-adduct 51 and a boron-mediated aldol reaction between enone 10 and aldehyde 9, exploiting unprecedented remote 1,6-stereoinduction, to give the (5S)-adduct 57.

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: the AB-spiroacetal segment

The convergent synthesis of the C1-C15 AB-spiroacetal subunit of altohyrtin A/spongistatin 1 is described. This highly stereocontrolled synthesis relies on matched boron aldol reactions of chiral methyl ketones, under Ipc(2)BCl mediation, to establish the C5, C9 and C11 stereocentres, and formation of the desired thermodynamic spiroacetal under acidic conditions. The scalable synthetic sequence developed provided access to multi-gram quantities of , thus enabling the successful completion of the total synthesis of altohyrtin A/spongistatin 1, as reported in Part 4.

Stereocontrolled total synthesis of (-)-callipeltoside A

[structure: see text] A highly stereocontrolled total synthesis of the cytotoxic macrolide (-)-callipeltoside A has been achieved in 23 steps (4.8% overall). Notable features include a novel asymmetric vinylogous aldol reaction to install the C13 stereocenter and (E)-trisubstituted alkene, an anti-selective aldol addition, a Sonogashira coupling, and, last, a Schmidt-type glycosylation to attach the sugar unit.

Synthesis of antimicrofilament marine macrolides: synthesis and configurational assignment of a C5-C16 degradation fragment of reidispongiolide A

Reidispongiolide A is a representative member of the sphinxolide/reidispongiolide group of cytotoxic 26-membered macrolides of marine origin. By interacting with actin in the cell cytoskeleton, the reidispongiolides and sphinxolides are potent microfilament destabilizing agents that represent a promising mechanism of action for developing novel anticancer drugs. An aldol-based synthesis of a library of diastereomers of C(8)-C(16) and C(5)-C(16) fragments and detailed NMR comparison with a reported degradation fragment enabled a configurational assignment for a major part of the reidispongiolide macrocyclic core, thus setting a solid foundation for ongoing synthetic efforts.

Toward the synthesis of reidispongiolide a: stereocontrolled synthesis of the C17-C22 and C23-C35 degradation fragments

[structure: see text] By relying on the asymmetric aldol reactions of chiral ketones, a highly stereocontrolled synthesis of each of the C(17)-C(22) and C(23)-C(35) degradation fragments of reidispongiolide A has been achieved. This permits a configurational assignment of the complete C(17)-C(36) region of this antimitotic macrolide, along with providing advanced intermediates for a projected total synthesis.

1,6-asymmetric induction in boron-mediated aldol reactions: application to a practical total synthesis of (+)-discodermolide

By relying solely on substrate-based stereocontrol, a practical total synthesis of the microtubule-stabilizing anticancer agent (+)-discodermolide has been realized. This exploits a novel aldol bond construction with 1,6-stereoinduction from the boron enolate of (Z)-enone 3 in addition to aldehyde 2. The 1,3-diol 7 is employed as a common building block for the C(1)-C(5), C(9)-C(16), and C(17)-C(24) subunits. [reaction--see text]

Toward the combinatorial synthesis of polyketide libraries: asymmetric aldol reactions with alpha-chiral aldehydes on solid support

[reaction: see text] The viability of performing stereocontrolled aldol additions with alpha-chiral aldehydes attached by a silyl linker to a hydroxymethylpolystyrene resin is demonstrated for boron and titanium enolates. Subsequent ketone reduction and manipulation on the solid support leads to elaborate stereopentad sequences, as occur in 6-deoxyerythronolide B and discodermolide.

Total synthesis of siphonarin B and dihydrosiphonarin B

The spirocyclic core of the siphonarins was constructed by a directed cyclization of a linear triketone, prepared using a Sn(II)-mediated aldol coupling and Swern oxidation at C9 and C13. To circumvent a facile retro-Claisen pathway generating a baconipyrone-type ester, a Ni(II)/ Cr(II)-mediated coupling reaction with vinyl iodide was used to complete the first synthesis of siphonarin B and dihydrosiphonarin B. A stable isomeric spiroacetal was also prepared which could not be equilibrated to the siphonarin skeleton.

A practical synthesis of (+)-discodermolide and analogues: fragment union by complex aldol reactions

A practical stereocontrolled synthesis of (+)-discodermolide (1) has been completed in 10.3% overall yield (23 steps longest linear sequence). The absolute stereochemistry of the C(1)-C(6) (7), C(9)-C(16) (8), and C(17)-C(24) (9) subunits was established via substrate-controlled, boron-mediated, aldol reactions of the chiral ethyl ketones 10, 11, and 12. Key fragment coupling reactions were a lithium-mediated, anti-selective, aldol reaction of aryl ester 8 (under Felkin-Anh induction from the aldehyde component 9), followed by in situ reduction to produce the 1,3-diol 40, and a (+)-diisopinocampheylboron chloride-mediated aldol reaction of methyl ketone 7 (overturning the inherent substrate induction from the aldehyde component 52) to give the (7S)-adduct 58. The flexibility of our overall strategy is illustrated by the synthesis of a number of diastereomers and structural analogues of discodermolide, which should serve as valuable probes for structure-activity studies.

Total Synthesis of the Callipeltoside Aglycon

Following macrolactonization, a Sonogashira coupling leads efficiently from 1 and 2 to the aglycon of the structurally unique cytotoxic macrolide callipeltoside A, isolated in tiny quantities from the lithistid sponge Callipelta sp. Key steps in the preparation of macrolide precursor 1 include a boron-mediated anti-aldol coupling (A) in tandem with Yamamoto's vinylogous aldol reaction (B). TES=triethylsilyl.